Surface treatment of tin-plated steel sheets

ABSTRACT

A surface treatment of a tin-plated steel sheet which comprises electrolyzing the tin-plated steel sheet as cathode with a quantity of electricity between 1 and 50 c/dm2 in an aqueous solution containing 1 to 50 g/l of at least one of aluminum dihydrogenphosphate, aluminum monohydrogenphosphate and their mixture.

United States Patent [1 1 Asano et al.

[451 Oct. 14, 1975 SURFACE TREATMENT OF TIN-PLATED STEEL SHEETS [30] Foreign Application Priority Data Aug. 1, 1973 Japan 48-86622 52 us. Cl 204/56 R; 204/35 R; 204/40 51 Int. cl. c251) 11/36 [58] Field of Search 204/35 R, 56 R, 54, 40

[56] References Cited UNITED STATES PATENTS 3,616,307 10/1971 Snyder ..204/56R FOREIGN PATENTS OR APPLICATIONS 698,632 11/1964 Canada 204/35 R Primary ExaminerR. L. Andrews Attorney, Agent, or Firm-Toren, McGeady and Stanger [57] ABSTRACT A surface treatment of a tin-plated steel sheet which comprises electrolyzing the tin-plated steel sheet as cathode with a quantity of electricity between 1 and 50 c/dm in an aqueous solution containing 1 to 50 g/l of at least one of aluminum dihydrogenphosphate, aluminum monohydrogenphosphate and their mixture.

2 Claims, No Drawings SURFACE TREATMENT OF TIN-PLATED STEEL SHEETS The present invention relates to surface treatment of tin-plated steel sheets, and more particularly relates to the surface treatment of tin-plated steel sheets for the purpose of prevention of damage to the sheet otherwise caused during a can-making process or during transportation.

conventionally, electro-tin-plated steel sheets are subject to a passivation treatment after electro-tin plating in order to prevent oxidation of the surface of the tin-plated steel sheet.

As for the passivation treatment, an electrolytic treatment in a sodium bichromate bath has been widely used. This electrolytic treatment, although very useful for preventing abnormal increase of the tin oxide, has a defect in that a small amount of chromium oxide in the surface adheres and accumulates on the piler and tools in the can-making line and causes damages to the surface of the tin-plated steel sheet.

In order to eliminate the above defect, lubricating oil is applied on the surface of the tin-plated steel sheet, but complete prevention of the surface damage can not be attained because the amount of the oil coating is limited for other reasons.

The surface damage deteriorates not only the outer appearance of a can but also its corrosion resistance. Therefore, there has been a strong demand for development of a complete method for preventing the surface damage which satisfies both the productivity and economy requirements.

The present inventors have conducted various studies and experiments on surface treatments which satisfy the oxidation (passivation) of tin and are effective to prevent the surface damage, and have discovered that the formation of a phosphate film is most effective for this purpose. Then the present inventors have con- The reason for limiting the concentration of aluminum dihydrogenphosphate Al (H PO aluminum monohydrogenphosphate Al (HPO or their mixture to the range from 1 to g/l is that if the concentration is less than 1 g/l, an effective film for preventing the surface damage is not formed and if the concentration is more than 50 g/l, there is a tendency for deterioration of the surface brightness other than the surface damage problem.

Meanwhile, the pH value of the aqueous solution is maintained to 3 or less since if the pH value exceeds 3, the surface brightness is deteriorated.

As for the bath temperature, better results can be obtained when it is between 20 and 80C.

When the tin-plated steel sheet is electrolyzed as a cathode under the above conditions, this treatment is performed with a quantity of electricity between 1 c/dm and 50 cldm When the quantity of electricity is less than 1 c/dm oxidation resistance of the tinplated steel sheet is unsatisfactory and on the other hand when it is larger than 50 c/dm the outer appearance of the tin-plated steel sheet deteriorates and solderability of the sheet is poor.

The tin-plated steel sheet, when treated under the above conditions, possesses very excellent properties.

The present invention will be more clearly, understood from the following examples.

EXAMPLE 1 A low carbon steel sheet of 0.23 mm thickness produced from an ordinary method was degreased in a caustic soda solution and then acid-pickled in a sulfuric acid solution. The surface-cleaned steel sheet was subjected to electro-tin-plating in a stannous phenolsulfonate electrolyte to give a tin coating of 12 g/m the thus tin-plated steel sheet heated to 270C to melt the tin to obtain mirror brightness, and then treated under the following conditions.

Treating bath:

Temperature:

Electrolysis conditions:

dUCIed f r her u ie 1 a m h f r f rming h 50 The tin-plated steel sheet was electrolyzed as acathode with a direct current.

EXAMPLE 2:

A tin-plated steel sheet prepared by the method of Example 1 was treated as a cathode under the following conditions.

Treating bath Temperature Electrolysis conditions Aluminum 30 g/l monohydrogcnphosphate Phosphoric acid 10 g/l Current density 8 A/dm Time 2 seconds EXAMPLE 3:

A tin-plated steel sheet prepared by the method of Example 1 was treated as a cathode under the following conditions.

Treating bath: Aluminum dihydrogenphosphatc l g/l A tin-plated steel sheet prepared by the method of Example 1 was treated as a cathode under the following conditions.

EXAMPLE 6:

A tin-plated steel sheet prepared by the same method as in Example 1 was treated as a cathode under the following conditions.

Treating bath Sodium bichromate 25 g/l pH Temperature 50C Electrolysis Current density 5 A/dm conditons:

Time 2 seconds Properties of the tin-plated steel sheets treated by the Treating bath: Aluminum monohydrogenphosphatc above examples are shown in Table 1. It IS clearly ung l dhydrogenphospham g 3/1 derstood from the table that the tin-plated steel sheets Temperature: 70C treated by the method according to the present inven- Elcctmlyss condmms g gg j 'nz tion show better oxidation resistance and remarkably better surface properties.

Table 1 Example Surfacc- Oxidation Soldcr- Lacquer- Yellow No. damage Resistance ability ability coloration Resistance Resistance g 1 o o o o o 2 5 3 0 4 o o o o o g 5 E s 5 8 We X o o o o Excellent 0 Good X Not good EXAMPLE 5':

A tin-plated steel sheet prepared by the same method as in Example 1 was treated as a cathode in a 0.5% sodium carbonate solution for 2 seconds with a current density of5 Aldm and then treated as a cathode under the following conditions.

Treating bath Aluminum 20 g/l monohydrogenphosphate Phosphoric acid 10 gl] pH 2 Temperature 70C Electrolysis conditions Current density 5 A/dm Time 2 seconds 

1. A SURFACE TREAMENT OF A TIN-PLATED STEEL SHEET WHICH COMPRISES ELECTROLYZING THE TIN-PLATED STEEL SHEET AS A CATHODE WITH A QUANTITY OF ELECTRICITY BETWEEN 1 AND 50 C/DM2 IN AN AQUEOUS SOLUTION CONTAINING 1 TO 50 G/L OF AT LEAST ONE OF ALUMINUM DIHYDROGENPHOSPHATE, ALUMINUM MONOHYDROGENPHOSPHATE AND THEIR MIXTURES, HAVING A PH OF 3 OR LESS.
 2. A surface treatment according to claim 1 in which the aqueous solution further contains phosphoric acid. 